Babies are born with 20 baby teeth already hidden in their jaws. A baby's first tooth breaks through the gum around 6 months old. It is most often one of the two bottom front teeth. The four upper front teeth tend to follow about one to two months later.

Teeth help us bite and chew our food. The sharp front teeth, called incisors, cut food when we bite. The molars in back are for grinding and chewing. Chewing breaks food into a soft ball that can be swallowed. It is the first step of digestion.

Teeth help you talk. Your tongue taps against your teeth or the roof of your mouth to make some sounds. Your teeth help shape words by controlling how air flows out of your mouth. Baby teeth help children speak clearly. Teeth also help shape our faces.

A tooth has a crown above the gum and a root below it. Look at your smile. The part you see is the crown.

Enamel is the hard outer coat of a tooth. It is the hardest part of the human body. It is harder even than bone.

Plaque is a sticky film of bacteria that keeps forming on your teeth. When you eat or drink sugary foods, bacteria in plaque make acid. The acid attacks the enamel, the hard outer coat of a tooth. A cavity is a little hole in a tooth.

Brush your teeth two times a day. Clean between your teeth every day with floss or a small brush made for the job.

Plaque you do not clean off each day can harden into tartar. Tartar is a rough substance that builds up along the gum line. Only a dentist can remove it, which is why cleaning every day matters.

A shark keeps growing new teeth all its life. Its teeth sit in rows. The front ones fall away, and fresh rows move forward like a conveyor belt. It can go through a huge number of teeth. Some sharks shed about 35,000 teeth during their lives.

People do not grow teeth that way. People grow two sets of teeth in a lifetime: baby teeth first, then adult teeth. Baby teeth fall out during childhood. The adult teeth that come next have to last for the rest of a person's life.

Words to know

• enamel: hard tooth coat
• plaque: sticky film
• tartar: hard crust
• incisors: sharp front teeth
• molars: back chewing teeth